Slack bridge: Update Slack bridge with Events API.

[PieterCK]

Currently, we use Slack's legacy RTM API as the "listener" for messages from Slack to Zulip. This PR updates our Slack Bridge to use the Webhook integration to send messages from Slack to Zulip.

We now utilize the Slack Webhook integration for this part of the bridge, which is being updated to use the latest Event API in PR#30465. Here are the key changes in this PR :

• Updated the documentation on how to set up the updated Slack Bridge.
• Removed the RTM API-based connection. Users can still use an older version of this repo if they want to maintain their existing connection.
• Made the bridge compatible with the Slack Webhook integration by preventing looping messages.
• Additionally, commit 5f42745 patches an unrelated linting error in salesforce.py that was causing tests to fail.

Fixes #825.

[PieterCK]

@zulipbot add "buddy review"

@kenneth please review this please, thank you

[zulipbot]

ERROR: Label "buddy review" does not exist and was thus not added to this pull request.

[PieterCK]

Update: Addressed review and dropped support for RTM API

[PieterCK]

@sbansal1999 I think this one's also ready for mentor review, please do check it out! Thanks

[PieterCK]

Now that PR#30465 is merged, this is ready for review!

[PieterCK]

This is ready for review!

[timabbott]

Merged, after some more rewriting of the docs and moving the documentation commit to the end.

Can you do some end-to-end testing of this end result and post about it in the #integrations channel? Once we've confirmed it's all happy, I can initiate a 9.4 server release and a release of this project to make this more readily available to end users.

[PieterCK]

Ok, I should be able to do that tomorrow. 👍